Modifying data which has been coded

ABSTRACT

Data (D), which has been coded by a coder (COD) so as to obtain coded data (CD), is modified. The data (D) may be, for example, a sequence of pictures which has been coded in accordance with an MPEG standard. The data (D), which is available in a coded form, is modified in the following manner. A partial decoder (PDEC) partially decodes the coded data (CD). That is, of a series of decoding steps (Sd( 1 ) . . . Sd(N)) which need to be carried out in order to decode the coded data (CD), the partial decoder (PDEC) carries out only a first few decoding steps (Sd( 1 ) . . . Sd(K)), with K and N being integers and K being smaller than N. Accordingly, partially decoded data (PDD) is obtained. A data-modifier (MOD) modifies the partially decoded data (PDD). Accordingly, modified partially decoded data (MPDD) is obtained. A complementary coder (CCOD) complementary codes the modified partially decoded data (MPDD). That is, the complementary coder (CCOD) carries out one or more coding steps (Sc(K) . . . Sc( 1 )), each of which is a complement (C) of a specific decoding step (Sd) which has been carried out by the partial decoder (PDEC). Accordingly, coded modified data (CMD) is obtained. Since only a partial decoding is carried out, fewer circuitry will be required than if the data (D) to be modified were fully decoded. Thus, the data (D) can be modified in a cost-efficient manner.

FIELD OF THE INVENTION

The invention relates to modifying data which has been coded. Theinvention may be applied, for example, to edit a sequence of pictureswhich has been coded in accordance with a standard defined by the MovingPictures Experts Group (MPEG).

BACKGROUND ART

The document ISO/IEC 13818-2 relates to the MPEG-2 standard. Section 7describes decoding steps for deriving a sequence of pictures from a datastream. These decoding steps include, amongst others, variable-lengthdecoding, inverse quantization, inverse discrete-cosine transformation,and motion compensation. A sequence of pictures is coded in accordancewith the MPEG-2 standard if a data stream is obtained which, by applyingthe decoding steps described in section 7, results in a correctlydecoded sequence of pictures.

SUMMARY OF THE INVENTION

It is an object of the invention to modify data which has been coded ina cost-efficient manner. The invention takes the following aspects intoconsideration. In principle, it is possible to fully decode the datawhich has been coded, to modify the data, and to code the modified data.However, in some applications, such an approach will be relativelycostly. This is the case, for example, if the data to be modified is asequence of pictures which has been coded in accordance with avideo-coding standard defined by the Moving Picture Experts GCroup(MPEG). MPEG coding or MPEG decoding of video data requires, amongstother things, a relative large data storage capacity for storingpictures. What is more, a full MPEG decoding followed by a full MPEGcoding, involves a further motion estimation and compensation inaddition to the motion estimation and compensation which have beencarried out at the coding end. This may introduce distortion.

In accordance with the invention, data which has been coded is modifiedin the following manner. The data which has been coded is decoded onlypartially. That is, only a portion of the steps which are needed tofully decode the data, is carried out. Accordingly, partially decodeddata is obtained. The partially decoded data is modified. The partiallydecoded data which has been modified, is coded so as to obtain codedmodified data. It should be noted that because, in the invention, amodification is carried out on partially decoded data, rather than onfully decoded data, some distortion may be introduced. However, in manyapplications, the distortion introduced will be relatively small suchthat, nevertheless, a satisfactory quality can be obtained at a decodingend. Since, in the invention, only a partial decoding is carried out,fewer circuitry will be required than if the data to be modified isfully decoded. Thus, the invention allows data which has been coded, tobe modified in a cost-efficient manner.

It has already been mentioned that the invention may be applied to edita sequence of pictures which has been coded in accordance with an MPEGstandard. In such an application, the partial decoding may comprisevariable length decoding, inverse quantization and inverse discretecosine transformation, but not motion compensation which requires arelatively large memory. As a result, blocks of prediction-error pixelswill be obtained instead of blocks of picture pixels. The blocks ofprediction-error pixels may be modified, for example, by adding blocksof auxiliary pixels to the blocks of prediction-error pixels, or byscaling the prediction-error pixels, or by filtering the blocks ofprediction-error pixcels, or by any combination the aforementionedoperations. The coding of modified blocks of prediction-error pixelscomprises discrete cosine transformation, quantization andvariable-length coding, but not motion estimation which is a complexoperation. Thus, motion vectors re-used. Optionally, the coding mayfurther comprise an error compensation for reducing any errors which maybe introduced by the discrete cosine transformation and the quantizationof a modified block of prediction-error pixels.

The invention and additional features which may be optionally used toimplement the invention to advantage, are apparent from and elucidatedwith reference to the drawings described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating basic features of theinvention as claimed in claim 1;

FIG. 2 is a conceptual diagram illustrating additional features asclaimed in 2;

FIG. 3 is a block diagram of an MPEG-video modifier in accordance withthe invention;

FIGS. 4a and 4 b are tables illustrating a series of fade coefficients afor carrying out a cross-fade between two different sequences ofpictures;

FIG. 5 is a block diagram of a picture processor which, when applied inthe MPEG-video modifier illustrated in FIG. 3, allows a cross-fade to becarried out between two different sequences of pictures;

FIG. 6 is a block diagram of an MPEG cross-fading assembly in accordancewith the invention;

FIGS. 7a and 7 b are conceptual diagrams illustrating an interlacedpicture having a 422 chrominance format and a 420 chrominance format,respectively.

FIG. 8 is a conceptual diagram illustrating a conversion from the 422chrominance format into the 420 chrominance format; and

FIG. 9 is a block diagram illustrating a picture processor which, whenapplied in the MPEG-video modifier illustrated in FIG. 3, allows aconversion to be carried out from the 422 chrominance format into the420 chrominance format.

DETAILED DESCRIPTION OF THE DRAWINGS

First, some remarks will be made on the use of reference signs. Similarentities are denoted with an identical letter code throughout thedrawings. In a single drawing, various similar entities may be shown. Inthat case, a numeral is added to the letter code, to distinguish similarentities from each other. The numeral will be between parentheses if thenumber of similar entities is a running parameter. In the descriptionand the claims, any numeral in a reference sign may be omitted if thisis appropriate.

FIG. 1 illustrates basic features of the invention in full lines. DataD, which has been coded by a coder COD so as to obtain coded data CD, ismodified in the following manner. A partial decoder PDEC partiallydecodes the coded data CD. That is, of a series of decoding steps Sd(1). . . Sd(N) which need to be carried out in order to decode the codeddata CD, the partial decoder PDEC carries out only a first few decodingsteps Sd(1) . . . Sd(K), with K and N being integers and K being smallerthan N. Accordingly, partially decoded data PDD is obtained. Adata-modifier MOD modifies the partially decoded data PDD. Accordingly,modified partially decoded data MPDD is obtained. A complementary coderCCOD complementary codes the modified partially decoded data MPDD. Thatis, the complementary coder CCOD carries out one or more coding stepsSc(K) . . . Sc(1), each of which is a complement C of a specificdecoding step Sd which has been carried out by the partial decoder PDEC.Accordingly, coded modified data CMD is obtained.

The following aspects have been taken into consideration with regard tothe basic features illustrated in FIG. 1. Since a modification iscarried out on the partially decoded data PDD rather than on fullydecoded data, some distortion may be introduced. Whether or not thedistortion is noticeable not only depends on the modification which iscarried out, but also depends, amongst other things, on what the codeddata CD represents and on how the data has been coded. For example, ifthe coded data CD represents a sequence of pictures, distortion oflow-frequency information will be more noticeable than distortion ofhigh-frequency information.

FIG. 2 illustrates the following features in addition to the featuresillustrated in FIG. 1. An auxiliary decoder ADEC fully decodes only aportion % of the coded data CD. That is, the auxiliary decoder ADEC, incombination with the partial decoder PDEC, carries out the completeseries of decoding steps for the portion % of the coded data CD.Accordingly, a piece of fully decoded data PFDD is obtained. Adistortion compensator CMP derives distortion-compensating data DCD fromthe piece of fully decoded data PFDD. An adder ADD adds thedistortion-compensating data DCD to the modified partially decoded dataMPDD. The complementary coder CCOD complementary codes the modifiedpartially decoded data MPDD with the distortion-compensating data DCDadded thereto. Accordingly, the coded modified data CMD is obtained.

The features illustrated in FIG. 2 provide the following advantageousaffects. The auxiliary decoder ADEC allows to fully decode a portion ofthe coded data CD which is critical in terms of distortion. For example,if the coded data represents a sequence of pictures, the auxiliarydecoder ADEC may fully decode a low-frequency portion of the coded dataCD. Since the thus obtained piece of fully decoded data PFDD is used tocompensate for distortion, a satisfactory quality can be obtained at adecoding end which, in some applications, would not be possible if nodistortion compensation were applied. It should be noted that asatisfactory quality can be achieved by fully decoding the coded data CDin its entirety which, however, is generally more expensive then fullydecoding a portion of the coded data CD only. Thus, the featuresillustrated in FIG. 2 allow a cost-efficient manner of data modificationwith

FIG. 3 shows a basic example of an MPEG-video modifier which recapturesthe characteristics illustrated in FIG. 1. The MPEG-video modifier maybe used for carrying out a picture processing between a coding end CEand a decoding end DE, both illustrated in broken lines. The coding endCE codes a sequence of pictures P in accordance with an MPEG standard.As a result, an MPEG data stream DS is obtained. The MPEG-video modifierprovides a modified MPEG data-stream MDS in response to the data streamDS. The decoding end DE which receives the modified MPEG data-stream MDSwill not reproduce the sequence of pictures P but rather a modifiedversion thereof. Thus, in effect, the MPEG-video modifier modifies videodata formed by sequence of pictures P.

The MPEG-video modifier shown in FIG. 3 comprises the following mainfunctional blocks: a prediction-error decoder PED, a picture processorPP and a prediction-error coder PEC. The prediction-error decoder PEDcomprises an input buffer IBUF, a variable-length decoder VLD,de-quantizer DQ1 and an inverse discrete cosine transformer IDCT1. Theprediction-error decoder DEC also comprises a motion-vector decoder MVD.The prediction-error coder PEC comprises a coding-error compensator CEC,a discrete-cosine transformer DCT, a quantizer Q, a variable-lengthcoder VLC and an output buffer OBUF. The coding-error compensator CECcomprises a subtractor SUB1, a de-quantizer DQ2, an inverse discretecosine transformer IDCT2, a subtractor SUB2, a memory MEM, and a motioncompensator MC.

The MPEG-video modifier illustrated in FIG. 3 basically operates asfollows. The prediction-error decoder PED derives blocks ofprediction-error pixels R from the MPEG data stream DS received. Theprediction-error decoder PED also provides motion vectors V which arecomprised in the MPEG data stream DS in a coded form. Each motion vectorV is associated to a specific block of prediction-error pixels R. Thepicture processor PP carries out a certain picture-processing function Fon the blocks of prediction-error pixels R. Accordingly, processedblocks of prediction-error pixels RP are obtained. The prediction-errorcoder PEC codes the processed blocks of prediction-error pixels RP. Tothat end it uses the motion vectors V. As a result, the modified MPEGdata stream MDS is obtained.

The prediction-error coder PEC will now be discussed in greater detail.The coding-error compensator CEC subtracts a coding-error compensatingblock of pixels EC from a processed block of prediction-error pixels RP.Accordingly, a coding-error compensated processed block ofprediction-error pixels RPC is obtained. The discrete-cosine transformerDCT transforms the coding-error compensated processed block ofprediction-error pixels RPC into a block of spatial-frequencycoeffients. The quantizer Q quantizes each spatial-frequency coefficientin the block. As a result, a block of quantized spatial-frequencycoefficients is obtained which is supplied the variable-length coderVLC. In response, the variable-length coder VLC provides variable-lengthcodes which are written into the output buffer OBUF and, after a certaintime, are read from the output buffer OBUF so as to form part of themodified MPEG data stream MDS.

The coding-error compensator CEC establishes error-compensating, blocksof pixels EC in the following manner. For each coding-error compensatedprocessed block of prediction-error pixels RPC, the inverse discretecosine transformer IDCT2 preceded by the de-quantifier DQ2, provides adecoded version RPCD of the coding-error compensated processed block ofprediction-error pixels RPC. The subtractor SUB1 subtracts the originalcoding-error compensated processed block of prediction-error pixels RPCfrom the decoded version RPCD thereof. As a result, a coding-error blockof pixels E is obtained which is stored in the memory MEM. Thus, whenthe prediction-error coder PEC receives a processed block ofprediction-error pixels RP belonging to a picture P(n), the memory MEMwill contain coding-error blocks of pixels E belonging to the previouspicture P(n−1). The motion compensator MC selects one of thesecoding-error blocks of pixels E on the basis of a motion vector Vassociated to the processed block of prediction-error pixels RP. Thecoding-error block of pixels E which has been selected constitutes theerror-compensating block of pixels EC.

The MPEG-video modifier illustrated in FIG. 3 may affect the picturequality to some extent. This is due to the fact that, in the MPEG-videomodifier illustrated in FIG. 3, picture processing is carried out onprediction-error pixels instead of on picture pixels. A block of picturepixels can be obtained by adding to a block of prediction-error pixelsR, a motion compensated block of picture pixels belonging to a previousimage. The MPEG-video modifier illustrated in FIG. 3 will provide apicture quality similar to that of an MPEG video modifier whichprocesses picture pixels, if the following condition is satisfied. Thesum of the results obtained by applying the picture-processing functionF to the block of prediction-error pixels R and to the motioncompensated block of picture pixels, respectively, is equal to theresult obtained by applying the picture-processing function F to the sumof the block of prediction-error pixels R and the motion-compensatedblock of picture pixels.

The MPEG-video modifier illustrated in FIG. 3 may be used, for example,to carry out a cross fade between a first sequence of picturesrepresented by the MPEG-coded data stream DS, and a second sequence ofpictures. Ideally, the modified MPEG-coded data stream MDS should thenrepresents a cross-faded sequence of pictures having the followingproperty:

PCF(n)=α(n)×P 1(n)+(1−α)×P 2(n)

With PCF being a picture from the cross-faded sequence of pictures, P1being a picture from the first sequence of pictures PS1, P2 being apicture from the second sequence of pictures PS2, α being a fadecoefficient which gradually decreases from 1 to 0 in a step-wise manneror vice versa, and n being an integer indicating a picture number of asequence. The fade coefficient α is constant throughout a picture, butmay vary from one picture to an other.

FIG. 4a and 4 b illustrate fade coefficients α for a series of pictures,each picture having a different picture number n. The only differencebetween FIG. 4a and 4 b is that, in FIG. 4a, the picture number n is inthe order in which the pictures are displayed whereas, in FIG. 4b, thepicture number n is in the order in which the pictures are coded. Beloweach picture number n=3.11, in the middle row titled CT, is a letter Bor P. The letter B or P indicates that the picture has been B-type orP-type coded, respectively, in accordance with the MPEG standard. Theorder in which the pictures are coded depends on the type of codingused. FIG. 4b shows that a picture of the B-type is always preceded by apicture which of the P-type having a higher picture number n.

FIG. 5 illustrates a picture processor PP which, when applied in theMPEG-video modifier illustrated in FIG. 3, allows a cross-fade to becarried out between a first and a second sequence of pictures. Theblocks of prediction-error pixels R and the motion vectors V which thepicture processor PP receives from the partial decoder PDEC shown inFIG. 3, represent the first sequence of pictures. In addition to eachblock of prediction-error pixels R, the picture processor also receivesa DC coefficient DC_R. The DC coefficient DC_R originates from the blockof spatial-frequency coefficients from which the inverse discrete cosinetransformer IDCT1, illustrated in FIG. 3, derives the block ofprediction-error pixels R. The picture processor PP further receivesblocks of picture pixels M belonging to the second sequence of pictures.The picture processor PP comprises the following elements: three scalingfunctions SC1, SC2, and SC3, two memories MEM1 and MEM2, two motioncompensators MC1 and MC2, three adders ADD1, ADD2 and ADD3, and asubtractor SUB.

The picture processor PP illustrated in FIG. 5 operates as follows. Fora block of prediction-error pixels R belonging to a picture P, thescaling function SC1 multiplies each prediction-error pixel with thefade coefficient α applying to the picture P. Accordingly, the scalingfunction SC1 provides blocks of scaled prediction-error pixels.

Due to the fact that, for the purpose of cross fading, prediction-errorpixels are processor has to carry out the cross fade illustrated inFIGS. 4a and 4 b. Let it further be assumed that the scaling functionSC1 receives a block of prediction-error pixels R belonging to thepicture number 8 which is a P-type coded picture. This block ofprediction-error pixels is a difference B(P8)−B(P5) between a block ofpixels B(P8) belonging to picture number 8, and a block of pixels B(P5)belonging to picture number 5. According to the cross fade illustratedin FIGS. 4a and 4 b, the pixels in the block B(P8) belonging to picturenumber 8 are to be multiplied by α=0.7, and the pixels in the blockB(P5) belonging to picture number 5 are to be multiplied by α=0.8.However, since in the picture processor illustrated in FIG. 5 thesepixels are not available as such, but only in a differential form, theycan not be scaled individually. As a result, some error is introduced.For example, if the scaling function SC1 applies cross-fade coefficientα=0.7 for the block of prediction-error pixels R being the differenceB(P5)−B(P5), it will provide a scaled difference 0.7×B(P8)−0.7×B(P5)whereas it should provide 0.7×B(P8)−0.8×B(P5).

To compensate the error described hereinbefore, the picture processorillustrated in FIG. 5 comprises an auxiliary decoder formed by the adderADD1, the motion compensator MC1 and the memory MEM1. The auxiliarydecoder provides, on the basis of successive DC coefficients DC_R,so-called DC approximations of blocks of picture pixels which would beobtained if the necessary decoding steps were carried out on the blocksof prediction-error pixels R.

A DC approximation is obtained in the following manner. The memory MEM1contains previous DC approximations belonging to a previous picturewhich, at the coding end, has served as a reference for establishing theblock of prediction-error pixels R concerned. Each previous DCapproximation is associated to a different block of pixels in theprevious picture. The motion compensator MC1 uses the previous DCapproximations to provide a prediction for the DC approximation on thebasis of the motion vector V. The adder ADD1 adds the prediction for theDC approximation to the DC coefficient DC_R associated to the block ofprediction-error pixels R. The result of this addition is stored in thememory MEM1 and constitutes the DC approximation which will be used forerror compensation. In effect, the adder ADD1, the motion compensatorMC1 and the memory MEM1, which constitute the auxiliary decoder, incombination with the partial decoder PEC illustrated in FIG. 3, form alow-resolution MPEG-video decoder. This low-resolution MPEG videodecoder fully decodes the MPEG video data stream DS using only theportion formed by the DC coefficients of the blocks of spatial-frequencycoefficients.

The scaling function SC2 reads from the memory MEM 1 the DCapproximation which is to be used for error compensation, and multipliesthe DC approximation with an appropriate error-compensation coefficient.The thus scaled DC approximation forms a block of error-compensatingpixels. The adder ADD2 adds the block of error-compensating pixels tothe block of scaled prediction-error pixels provided by the scalingfunction SC1. Accordingly, the adder ADD2 provides an error-compensatedblock of scaled prediction-error pixels.

The following is an example of the error compensation describedhereinbefore. Let it be assumed that the scaling function SC1 providesthe block of scaled prediction-error pixels being the scaled difference0.7×B(P8)−0.7×B(P5) discussed hereinbefore. The scaling function SC2reads from the memory MEM1 a DC approximation DC_B(P5) of the blockB(P5). The DC approximation DC_B(P5) has been established earlier on thebasis of a DC coefficient DC_R belonging to picture number 5. Thescaling function SC2 multiplies the DC approximation by −0.1 so as toform a block of error-compensating pixels −0.1×DC_B(P5) which the adderADD2 adds to the block of scaled prediction-error pixels being thescaled difference 0.7×B(P8)−0.7×B(P5). Accordingly, the adder ADD2provides the following error-compensated block of scaledprediction-error pixels: 0.7×B(P8)−0.7×B(P5)−0.1×DC_B(P5). The latter isan approximation of what ideally should have been obtained:0.7×B(P8)−0.8×B(P5).

As mentioned hereinbefore, the picture processor PP receives blocks ofpicture pixels M belonging to the second sequence of pictures which isto be cross-faded with the first sequence of pictures represented by theblocks of prediction-error pixels R. The scaling function SC3 multiplieseach picture pixel with a complementary fade coefficient 1−α.Accordingly, the scaling function SC3 provides blocks of scaled picturepixels.

The memory MEM2, the motion compensator MC2 and the subtractor SUB forman insertion-adaptation path which processes the blocks of scaledpicture pixels. The blocks of scaled picture pixels are written into thememory MEM2 such that when a certain picture of the second sequence isprocessed, the memory MEM2 contains a faded version of the previouspicture. The motion compensator MC2 selects, on the basis of the motionvector V, a reference block of scaled picture pixels from the fadedversion of the previous picture stored in the memory MEM2. Thesubtractor SUB subtracts the reference block of scaled picture pixelsfrom the block of scaled picture pixels provided by the scaling functionSC2. Accordingly, the subtractor SUB provides an insertion-adapted blockof scaled picture pixels. It is noted that applicant's co-pending patentapplication 09/400960 describes an insertion-adaptation path forprocessing video which is to be added to partially decoded MPEG video.

The adder ADD3 adds an insertion-adapted block of scaled picture pixelsto the error-compensated block of scaled prediction-error pixelsprovided by the adder ADD2. Accordingly, the adder ADD3 provides across-faded block of pixels RP. The prediction-error coder PECillustrated in FIG. 3, codes the cross-faded block of pixels RP.Accordingly, the modified MPEG data stream MDS represents the cross-fadebetween the first sequence of pictures, which is represented by theMPEG-video stream DS, and the second sequence of pictures which issupplied to the picture processor illustrated in FIG. 5.

FIG. 6 illustrates an MPEG-video cross-fading assembly. The MPEG-videocross-fading assembly carries out a cross fade between a first sequenceof pictures represented by a first MPEG data stream DS1, and a secondsequence of pictures represented by a second MPEG data stream DS2.Accordingly, a cross-faded MPEG data stream CFDS is obtained whichrepresents a cross-fade between the two picture sequences. The MPEGcross-fading assembly comprises two delay circuits DEL1, DEL2, twodecoders DEC1, DEC2, two MPEG-video modifiers MOD1, MOD2, and a switchSW. Each of the two MPEG video modifiers MOD1, MOD2 is similar to theMPEG video modifier shown in FIG. 3 comprising the picture processor PPillustrated in FIG. 5.

The MPEG-video modifier illustrated in FIG. 6 operated as follows. Thedelay circuits DEL1, DEL2 delay the first and the second MPEG datastreams DS1, DS2, respectively, to compensate for delays in the decodersDEC1, DEC2. Decoder DEC1 provides a fully decoded first sequence ofpictures PS1 and decoder DEC2 provides a fully decoded second sequenceof pictures PS2. MPEG-video modifier MOD1 carries out a cross-fadebetween the first sequence of pictures represented by the first MPEGdata stream DS1 and the fully decoded second sequence of pictures PS2.MPEG-video modifier MOD2 carries out a complementary cross-fade betweenthe fully decoded first sequence of pictures PS2 and the second sequenceof pictures represented by the second MPEG data stream DS2. That is, thesum of the fade coefficients used in the MPEG-video modifiers MOD1, MOD2is 1. For example, when the MPEG-video modifier MOD1 applies thefade-coefficient α1=0.9, the MPEG-video modifier MOD2 applies thefade-coefficient α2=0.1.

Thus, each MPEG-video modifier MOD1, MOD2 provides a modified MPEG-videodata steam MDS1, MDS2, respectively, which, in principle, represents thesame cross-faded sequence of pictures. If the fade coefficient α1applied by the MPEG-video modifier MOD1 is greater than 0.5, the switchSW selects the modified MPEG data stream MDS1 to constitute thecross-faded data stream CFDS. Conversely, if the fade coefficient α2applied by the MPEG-video modifier MOD2 is greater than 0.5, the switchSW selects the modified MPEG data stream MDS2 to constitute thecross-faded data stream CFDS.

The reason for switching between the modified data streams MDS1 and MDS2as described hereinbefore, is as follows. Referring to the pictureprocessor illustrated in FIG. 5, the insertion-adaptation path formed bythe memory MEM2, the motion compensator MC2, and the subtractor SUB,processes video information on the basis of motion vectors V which donot belong to that video information. This may distort the videoinformation to some extent. If the fade coefficient α is relativelyhigh, which implies that 1−α is relatively low, the somewhat distortedvideo information will contribute to the cross-faded sequence ofpictures to a relatively small extent only. However, if the fadecoefficient α is relatively low, which implies that 1−α is relativelyhigh, the somewhat distorted video information will significantlycontribute to the cross-faded sequence of pictures. The switch SWselects the modified data stream MDS1 or MDS2 which has been obtainedwith the greater fade coefficient α. Thus, the switch selects themodified data stream MDS1 or MDS2 which has the better picture quality,to constitute the cross-faded MPEG data stream CFDS.

FIG. 7a illustrates an interlaced picture in a 422 chrominance format asdefined in the MPEG-2 standard. FIG. 7b illustrates an interlacedpicture in a 420 chrominance format. In FIGS. 7a and 7 b, a full linerepresents chrominance pixels belonging to an odd line of a picture, anda broken line represents chrominance pixels belonging to an even line.In the 422 chrominance format, there are twice as many chrominancepixels in the vertical direction as in the 420 chrominance format.

FIG. 8 illustrates a conversion of an interlaced picture in the 422chrominance format into an interlaced picture in the 420 chrominanceformat, which conversion will be referred to as a 422/420 conversionhereinafter. In FIG. 8, a chrominance pixel CP is represented by a smallcircle. In the 422/420 conversion illustrated in FIG. 8, a verticalfilter VFIL provides a chrominance pixel for the 420 chrominance formatby making a weighed combination of the best corresponding chrominancepixel in the 422 format and other chrominance pixels. These otherchrominance pixels have the same horizontal position as the bestcorresponding chrominance pixel, but belong to neighboring lines havingthe same parity as the line to which the best corresponding chrominancepixel belongs.

FIG. 9 illustrates a picture processor PP which can be applied in theFIG.3 MPEG-video modifier to carry out a 422/420 conversion. The pictureprocessor PP receives from the partial decoder illustrated in FIG. 3,blocks of prediction-error chrominance pixels R(C) and the motionvectors V associated thereto. With each block of prediction-errorchrominance pixels R(C), the picture processor also receives codingparameters PAR which are included in the MPEG data stream. The codingparameters PAR indicate how the block of prediction-error chrominancepixels R(C) has been obtained at the coding end. The picture processorcomprises the following elements: a memory MEM, a vertical filter VFILand a conversion controller CON.

The picture processor operates in the following manner. The blocks ofprediction-error chrominance pixels R(C) and the motion vectors V arewritten into the memory MEM such that the vertical filter VFIL is ableto gather prediction-error chrominance pixels from different blockswhich are each other's neighbors in the vertical direction, as well asthe motion vectors belonging to these blocks. For a certainprediction-error chrominance pixel, the conversion controller CON checksif the block to which the prediction-error chrominance pixel belongs,and the blocks which are upper neighbor and lower neighbor,respectively, have similar motion vectors V. If this is the case, thevertical filter VIGIL uses these blocks to provide a prediction-errorchrominance pixel for the 420 format. If not, the vertical filter VFILuses the block to which the prediction-error chrominance pixel belongsonly and, in effect, mirrors this block to simulate an upper neighborand a lower neighbor providing filter input data.

The conversion controller CON verifies, on the basis of the codingparameters PAR and the motion vector V, if the following threeconditions apply. Firstly, a frame-type prediction mode has been used toestablish the block of prediction-error chrominance pixels R(C)concerned. Secondly, the motion vector V is expressed in full pixelunits or, when it is expressed in half pixel units, its verticalcomponent has an even value. Thirdly, the parity of the motion's vectorvertical component changes when divided by 2. If these three conditionsapply, the conversion controller CON changes the frame-type predictionmode into a field-type prediction mode. To that end, it calculates abinary value for a flag motion_vertical_field_select included in themodified MPEG data stream MDS. At a decoding end, this flag selectseither a field comprising even lines or a field comprising uneven linesfor carrying out a motion compensation. The binary value for the flagmotion_vertical_field_select is such that, in the 422/420 conversion,line parity is preserved. That is, it ensures that if, for aprediction-error chrominance pixel on a line in an interlaced picture,the motion vector V refers to a chrominance pixel on an other line inanother interlaced picture, the line which is referred to has the sameparity in the 422 chrominance format and in the 420 chrominance format.

There are numerous ways of physically spreading functions or functionalelements over various units. In this respect, the drawings are verydiagrammatic, each representing only one possible embodiment of theinvention. Thus, although a drawing shows different functional elementsas different blocks, this no by means excludes that some functionalelements, or all functional elements, may be implemented as a singlephysical unit.

Although, in FIG. 2, the auxiliary decoder ADEC fully decodes a portion% of the coded data CD in combination with the partial decoder PDEC, itis also possible for the auxiliary decoder ADEC to fully decode aportion % of the coded data CD by itself. In that case, the auxiliarydecoder ADEC will carry out all the decoding steps Sd(1) . . . Sd(N) fora portion of the coded data CD only, rather than applying the missingdecoding steps to the partially decoded data PDD as illustrated in FIG.2.

Although, in FIG. 6, the decoders DEC and the MPEG video-modifiers MODare shown as separate blocks, they may share one or more circuits andfunctions. As mentioned hereinbefore, each MPEG video-modifier MODcomprises a picture processor PP similar to the picture processorillustrated in FIG. 5. The latter comprises an auxiliary decoder formedby the adder ADD1, the motion compensator MC1 and the memory MEM1. Theseelements may be shared with the decoder DEC receiving the same datastream DS as the MPEG-video modifier MOD concerned. For example, theadder ADD2 in the picture processor PP illustrated in FIG. 5 may receiveerror-compensating block of pixels which are derived from the decodedsequence of pictures PS1 or PS2. Since, in that case, theerror-compensating blocks of pixels are derived from a fully decodedpicture rather than a DC approximation thereof, it is possible fullycompensate any error in the block of scaled prediction-error pixelsprovided by the adder ADD1.

With respect to the picture processor PP illustrated in FIG. 5, thefollowing should be noted. It is possible to modify the pictureprocessor PP such that it is suitable for inserting video data into anMPEG data stream. The video data to be inserted in the MPEG data streammay be, for example, a logo. In such a modification of the pictureprocessor PP illustrated in FIG. 5, the blocks of prediction-errorpixels R belong to the MPEG data stream in which video data needs to beinserted. The blocks of picture pixels M belong to the video data to beinserted. If the video data to be inserted is a logo, there will berelatively many blocks of picture pixels M with zero pixels only,because the logo will only occupy a relatively small portion of a fullpicture.

The following modifications make the picture processor PP illustrated inFIG. 5 suitable for video insertion. The adder ADD1, the motioncompensator MC1, the memory MEM1 and the scaling function SC2 areremoved. The scaling functions SC1 and the adder ADD2 are replaced byshort circuits. The scaling function SC3 may also be replaced by a shortcircuit but, preferably, it is replaced by a processing arrangement forprocessing the video to be inserted. This processing may include acalculation of average pixel values belonging MPEG data stream and acorrection of the video to be inserted on the basis of these averagepixels values. Such processing takes into account, as it were, what willhappen with the processed blocks of prediction error pixels RP at adecoding end. Accordingly, a satisfactory approximation be achieved of avideo insertion in which the MPEG data stream is fully decoded. Theinternational application PCT/IB99/00235 incorporated by referenceherein, describes processing of video to be inserted which allows such asatisfactory approximation.

With regard to video insertion and the modifications describedhereinbefore, it should be noted that the adder ADD3 may be replaced byan intelligent switch. The intelligent switch, in effect, detects foreach pixel in the processed block of prediction-error pixels RP, whetherthere is a non-zero pixel in the block of picture pixels M. That is, itdetects whether an output pixel should be an inserted-video pixel. Ifso, the intelligent switch selects the output of the subtractor SUB toprovide the output pixel. If not, the intelligent switch selects therelevant pixel of the block of prediction-error pixels R to provide theoutput pixel.

Finally, it should be noted that the prediction-error decoder PED in theMPEG-video modifier illustrated in FIG. 3, does not only provide blocksof prediction-error pixel R, but also blocks of picture pixels if thepicture concerned has been I-type coded at the decoding end DE. In thatcase, the coding-error compensator CEC may be disabled. Furthermore,since the picture processor PP will process picture pixels rather thanprediction-error pixels, the picture processing function F can, inprinciple, be carried out without introducing any distortion.

What is claimed is:
 1. A method of modifying data (D), the data (D)having been coded so as to obtain coded data (CD), the method comprisingthe steps of: partially decoding (PDEC) the coded data (CD) so as toobtain blocks of prediction-error pixels; modifying (MOD) the blocks ofprediction-error pixels so as to obtain modified partially decoded data(MPDD); and complementary coding (CCOD) the modified partially decodeddata (MPDD), so as to obtain coded modified data (CMD).
 2. A method ofmodifying data as claimed in claim 1, the method comprising the stepsof: fully decoding (PDEC,ADEC) a portion (%) of the coded data (CD) soas to obtain a piece of fully decoded data (PFDD); derivingdistortion-compensating data (DCD) from the piece of fully decoded data(PFDD); adding the distortion-compensating data (DCD) to the modifiedpartially decoded data (MPDD); and complementary coding (CCOD) themodified partially decoded data (MPDD) with the corrective data (CRD)added thereto, so as to obtain the coded modified data (CMD).
 3. Adata-modifying assembly for modifying data (D), the data (D) having beencoded so as to obtain coded data (CD), the data-modifying assemblycomprising: a partial decoder (PDEC) for partially decoding the codeddata (CD) so as to obtain blocks of prediction-error pixels; a datamodifier (MOD) for modifying the blocks of prediction-error pixels so asto obtain modified partially decoded data (MPDD); and a complementarycoder (CCOD) for complementary coding the modified partially decodeddata (MPDD), so as to obtain coded modified data (CMD).